The present invention relates to measurements in machinery and, more particularly, to measurement of proximity of one element in a machine apparatus to another without actually touching one of the elements.
Proximity probes, for example, are commonly used in machinery for determining the condition of the machinery and transmitting signals related thereto to indicating or recording apparatus remote from the point of measurement. The remote feature has considerable importance in apparatus such as, for example, pumps, motors and axial compressors where access to the area to be measured is unavailable except pursuant to a substantial disassembly of the apparatus. If a proximity probe fails, either due to damage during manufacturing or during its operating life, the purpose for inclusion of such a proximity probe is defeated since disassembly is required to determine the condition of the machinery as well as to replace the failed proximity probe. Although many different types of apparatus are users of proximity measurement probes and the present invention is equally related thereto, a specific example of a large heavy duty gas turbine is employed for concreteness of description. The background problems in other types of apparatus are similar to the example given and further description of such apparatus is omitted.
In a large gas turbine, it is important to monitor the radial vibration of the rotor in its journal bearings to detect levels of radial vibration which may provide an indication of bearing damage or of equipment malfunction which may be related to excessive imbalance in the rotating equipment. Typically, when a rotor journal is vibrated due to imbalance, its center describes a circular or elliptical orbit. It has thus been customary to position a pair of proximity sensors spaced ninety degrees apart with their axis oriented radially to detect the journal motion along orthogonal axis. The types of sensors used in such applications have customarily been proximity sensing devices of the type having a sensing coil at their extremities. In order to obtain interpretable results from such proximity sensors, the sensing coil must be accurately positioned with respect to the body of the apparatus and/or a known position of the surface being monitored. Where direct radial access to a journal probe is available in a fully assembled gas turbine, initial adjustment of a proximity probe, either on original apparatus installed during manufacture or a field installation to replace a defective probe, is relatively straightforward. For example, U.S. Pat. No. 4,018,083, of common assignee with the present invention, discloses a radial probe positioning apparatus in which a probe holder includes a stop for pre-positioning the probe a predetermined distance from a rotating shaft of a turbine.
When radial access is denied due to surrounding equipment such as housings etc., radial adjustment of a replacement probe may be feasible only through major disassembly of the gas turbine. The fundamental economic reason for employing such sensor probes is their value in providing material for an analysis of the condition of the gas turbine without major disassembly and the attendant costs. Such analysis is an on-going procedure throughout the life of the gas turbine and its value increases with the years of use. Unfortunately, proximity sensors are relatively subject to damage during manufacture of the apparatus and also are subject to field failure during use of the gas turbine. As a result, if access for replacement and adjustment of radial sensors is unavailable, after an extended period of use when their value becomes maximum, they can be in a failed condition and unavailable to provide the necessary input to the decision process without the unnecessary substantial disassembly of the gas turbine which they were intended to avoid.
One apparatus disclosed in U.S. Pat. No. 4,047,103 slides a sensor down a curved tube for eddy current measurement of position for measuring displacement of core components of a liquid metal nuclear reactor. Beyond positioning of a sensor along a curved connecting path, this invention offers little solution to the problems discussed in the preceding.
A further type of measurement includes the measurement of axial displacement of a thrust bearing. In some machinery, axial vibration is a factor requiring measurement. In other applications, the average axial position is of primary interest so that an assessment of lubricant film thickness can be obtained to evaluate possible thrust bearing damage due to excessive axial load or motion. In either case, a proximity probe must be positioned a predetermined distance from a nominal position of a target surface with the axis of the probe parallel to the axis of the apparatus. Due to manufacturing tolerances in the length relationships of the shaft, thrust bearing and compressors, the absolute positioning of a proximity sensor probe can not be established until the gas turbine is assembled and appropriate shims installed during manufacture. Once the final axial position of a thrust bearing is established, the proximity sensor must be adjusted to the required position. Adjustment of the proximity sensor customarily requires the disassembly of the bearing housing to gain access to the shaft collar and the thrust bearing.
Furthermore, during manufacturing, the proximity sensors are installed and their electrical cables for remote signal transmission are brought out of the bearing housing at the same time as the components of the apparatus are being assembled. The probe and its cable are relatively fragile elements which are at substantial risk during the assembly of the large, heavy and unwieldy components of the bearing housing assembly. Sufficient damage to the proximity sensor and/or its electrical cable, requiring an expensive disassembly, replacement and reassembly of the apparatus is required when damage occurs during manufacturing.
Even when relatively free access is available to the vicinity of the thrust bearing, replacement and adjustment of a proximity sensor is not necessarily possible. That is, direct axial access to the proximity sensor location is usually not provided and can not be obtained in a fully assembled equipment.